This invention relates to a process for producing prepolymers, which contain isocyanate terminal groups and which have a very low content of free isocyanate starting materials, from polyester polyols and polyisocyanates.
Prepolymers which contain isocyanate terminal groups and which are formed from polyisocyanates and polyesters which contain hydroxy groups are produced and used industrially on a large scale for very different purposes, e.g. for the production of cast PU elastomers.
For environmental reasons it is desirable to produce prepolymers which contain isocyanate groups and which only have a low content of unreacted polyisocyanates. This is all the more desirable since these prepolymers are often processed further at elevated temperatures to form final products, whereupon unreacted isocyanate starting materials can volatilise. Due to the toxicity of the polyisocyanates used, particular safety precautions are then necessary during the processing of the prepolymers, which increases the cost of processing prepolymers such as these to form the final products thereof.
According to the prior art, prepolymers such as these are sometimes produced by separating the prepolymers from unreacted isocyanate by distillation under vacuum after the reaction of the monomeric isocyanate starting material with the polyol. When polyester prepolymers are employed, however, significant amounts of isocyanate still remain in the prepolymer, even after distillation.
The object of the present invention is therefore to produce prepolymers which contain isocyanate terminal groups and which have a reduced content of monomeric isocyanate starting material compared with the prior art.
Surprisingly, it has now been found that prepolymers such as these can be produced by very substantially freeing the polyester polyols, before they are used, from free polyester components of low molecular weight which contain hydroxyl groups, such as diols and triols, and thus achieving a defined content of low molecular weight components in the polyols to be used.
The present invention therefore relates to a process for producing prepolymers which contain isocyanate terminal groups, which is characterised in that polyisocyanates in excess are reacted with a polyester which contains at least two hydroxy groups, of molecular weight 400 to 5000, wherein the polyester used has a content of polyols on which the polyesters are based of xe2x89xa66 mol %.preferably xe2x89xa63 mol % and most preferably xe2x89xa61.5 mol % with respect to the molar distribution of the polyester, and the reaction mixture is subsequently subjected to distillation, optionally with the addition of an entraining agent.
The polyesters which are preferably used in the process according to the invention have a theoretical content of 2 to 4 hydroxyl groups and a molecular weight of 400 to 5000, particularly 600 to 2500, and have a content of polyols (glycol components) on which the polyesters are based of xe2x89xa66 mol %, preferably xe2x89xa63 mol % most preferably xe2x89xa61.5 mol %.
According to the invention, the polyisocyanates are used in an equivalent ratio to the polyesters of at least 1.9:1, preferably from 2:1 to 20:1, most preferably from 2.2:1 to 10:1.
The content according to the invention of polyols on which the polyesters are based can be achieved according to the prior art by various routes, for example by flash evaporation or thin film distillation, whereupon after distillation the polyesters are quenched very rapidly, by intensive cooling, to a temperature less than 140xc2x0 C., preferably less than 100xc2x0 C., most preferably less than 70xc2x0 C., so as to prevent the establishment of an equilibrium comprising the presence of monomeric components. Corresponding methods are described in EP 0579988.
All known aliphatic, cycloaliphatic, araliphatic, aromatic and/or heterocyclic polyisocyanates can be used as polyisocyanates in the process according to the invention, such as those described, for example, by W. Siefken in Justus Liebigs Annalen der Chemie, 562, pages 75 to 136, with the proviso that it must be possible to distil these polyisocyanates under vacuum without decomposition. In general, polyisocyanates which are readily available industrially are most preferably used, such as toluene 2,4- and/or 2,6-diisocyanate (TDI).
The aforementioned polyesters which contain at least two hydroxy groups and which have molecular weights ranging from 400 to 5000 can be used as polyesters in the process according to the invention. Polyesters such as these, which contain hydroxy groups, are known for the production of homogeneous and cellular polyurethanes, and are described in DE-A 28 32 253 for example.
Polyesters which are particularly suitable are those which can be synthesised from commercially available diols and/or triols, which preferably contain 2 to 6 carbon atoms, as the polyol component, such as ethylene glycol, diethylene glycol, propanediol, butanediol or hexanediol, wherein the carbon chain can also contain oxygen bridges such as that in diethylene glycol, and from customary dicarboxylic acids, preferably adipic acid, or industrial mixtures thereof with succinic and/or glutaric acid, as the polycarboxylic acid component.
The polyesters which are used can be modified according to the prior art, or can incorporate additives such as plasticisers, emulsifiers or catalysts.
After the reaction of the polyisocyanates which are used with the polyesters which are used according to the invention, the reaction mixture is distilled, optionally with the addition of an entraining agent.
Distillation is usually conducted at temperatures within the range from about 130xc2x0 to 220xc2x0, at pressures which range from about 0.001 bar to 1 bar.
Entraining agents which are known for purposes such as this, for example toluene, can be used as entraining agents.
The most favourable amount of entraining agent to use can easily be determined by appropriate preliminary tests, and usually ranges from 2 to 5% by weight with respect to the amount of prepolymers.
Prepolymers which contain isocyanate terminal groups, which are produced by the process according to the invention and which have a very low content of isocyanate starting materials, can be further processed in the usual manner according to the prior art for the production of polyurethane plastics (using corresponding polyol synthesis components).